As is well-known in the industry, hydrocarbons are recovered from subterranean reservoirs by drilling a borehole (wellbore) into the reservoir. Such boreholes are commonly drilled using a rotating drill bit attached to the bottom of a drilling assembly (which is commonly referred to in the art as a bottom hole assembly or a BHA). The drilling assembly is commonly connected to the lower end of a drill string including a long string of sections (joints) of drill pipe that are connected end-to-end via threaded pipe connections. The drill bit, deployed at the lower end of the BHA, is rotated by rotating the drill string from the surface and/or by a mud motor deployed in the BHA. Mud motors are also commonly utilized with flexible, spoolable tubing commonly referred to in the art as coiled tubing. During drilling a drilling fluid (referred to in the art as mud) is pumped downward through the drill string (or coiled tubing) to provide lubrication and cooling of the drill bit. The drilling fluid exits the drilling assembly through ports located in the drill bit and travels upward, carrying debris and cuttings, through the annular region between the drilling assembly and borehole wall.
In recent years, directional control of the borehole has become increasingly important in the drilling of subterranean oil and gas wells, with a significant proportion of current drilling activity involving the drilling of deviated boreholes. Such deviated boreholes often have complex profiles, including multiple doglegs and a horizontal section that may be guided through thin, fault bearing strata, and are typically utilized to more fully exploit hydrocarbon reservoirs. Deviated boreholes are often drilled using downhole steering tools, such as two-dimensional and three-dimensional rotary steerable tools. Such tools commonly include a plurality of independently operable blades (or force application members) that are disposed to extend radially outward from a tool housing into contact with the borehole wall. The direction of drilling may be controlled by controlling the magnitude and direction of the force or the magnitude and direction of the displacement applied to the borehole wall. In rotary steerable tools, the housing is typically deployed about a rotatable shaft, which is coupled to the drill string and disposed to transfer weight and torque from the surface (or from a mud motor) through the steering tool to the drill bit assembly.
Directional wells are also commonly drilled by causing a mud motor power section to rotate the drill bit through a displaced axis while the drill string remains stationary (non-rotating). The displaced axis may be achieved, for example, via a bent sub deployed above the mud motor or alternatively via a mud motor having a bent outer housing. The bent sub or bent motor housing cause the direction of drilling to deviate (turn), resulting in a well section having a predetermined curvature (dogleg severity) in the direction of the bend. A drive shaft assembly deployed below the power section transmits downward force and power (rotary torque) from the drill string and power section through a bearing assembly to the drill bit. Common drive shaft assemblies include a rotatable shaft (mandrel) deployed in a housing.
The non-rotating sections (e.g., the above described housings) commonly include MWD and/or LWD sensors, electronic components and controllers, and electrical actuators (e.g., solenoids used to control steering blades). In the above described drilling assemblies a gap typically exists between the rotating and non-rotating sections (e.g., between the shaft and housing). Thus electrical power must be stored and/or generated in the non-rotating section or transferred across the gap from the rotating section to the non-rotating section. Moreover, in order to provide electronic communication between the rotating and non-rotating sections, data must also be transferred back and forth across the gap.
Techniques for transmitting electrical power and electronic data across the gap between rotating and non-rotating tool sections are known in the art. For example, sealed slip rings are conventionally utilized. While slip rings are known to be commercially serviceable, failure of certain slip ring components is a known cause of downhole tool failure. For example, slip ring seals have been known to fail, which can result in a loss of communication with the tool and the need to trip out of the borehole. Loss of electrical contact between the slip ring contact members is also a known cause of tool failure.
Inductive coupling devices are also known for transferring power and/or data between rotating and non rotating tool sections. For example, U.S. Pat. No. 6,540,032 to Krueger discloses an inductive coupling for transferring power and data between rotating and non-rotating sections of a downhole drilling assembly. While inductive coupling devices are known in commercial oilfield applications, there remains a need for improved devices for non-contact transmission of data and electrical power between tool sections. For example, inductive couplings tend to occupy a large physical space and are typically expensive to fabricate (due to the use of a wound magnetic core). Inductive couplings also tend to exhibit low transmission efficiencies owing to the relatively large gap between transmitter and receiver. Owing to the demand for smaller diameter and less expensive rotary steerable tools (and downhole tools in general), there is a need for improved non-contact power and data transmission devices.